- I was surprised that they had already decided what to do.
- I was surprised that they had decided what to do already.
Which sentence is correct?
Already normally refers to something that is in the present or recent past. It is mainly used in questions and affirmative sentences and usually expresses surprise that something has happened sooner than expected.
already - position in sentence
Note from the above examples that already usually occupy mid position in the clause.
So your first sentence is more appropriate.